This invention relates to an image input apparatus for converting an image into digital information.
Image input apparatus have been used to convert an image into digital information for application to an image processing unit or the like. For this purpose, such a conventional image input apparatus includes a drum scanner or an image scanner. The drum scanner projects a light spot on an image source of a photographic paper sheet or a photographic film wound around a drum rotating about its axis. The drum scanner employs a photosensor sensitive to the intensity of the light reflected on or transmitted through the image source. Each time the drum makes a full rotation about its axis, one scan line segment comprised of a series of picture elements (pixels) is inputted and the light spot and the photosensor are moved in a direction normal to the direction of rotation of the drum in order to input the next scan line segment. The image scanner employs a linear charge coupled device (CCD) placed at a position facing to an image source for inputting one scan line segment. The image source is moved with respect to the linear CCD in a direction normal to the direction of scanning of the linear CCD in order to input the next scan line segment.
One problem with these conventional scanners is that its scanning speed is limited because the scanning operation is made in a mechanical manner. This is true particularly when the image is required to be scanned with a high resolution. Also these scanners will be difficult to keep the high resolution and the high accuracy because vibration occur according to the mechanical scanning operation. Additionally, as undesirable effects on the image conversion process with the drum scanner, a slack would occur on the image source wound around the drum. And with the image scanner, it is difficult to scan a large image with a high resolution since it has a limited number of pixels.
Such conventional image input apparatus have been designed to take one scan line segment as the unit of the image. Although this design is satisfactory for inputting the whole area of the image, it is impractical for the case where only a portion of the image is required to be inputted. In this case, conventional image input apparatus require a great number of memory sections for storing unnecessary information regarding the unnecessary portions of the image. This is true particularly when the image is processed with a high resolution and/or when the image has a large size.
In order to process a large number of image information, it is the current practice to transfer a part of the image information stored in an external memory into a computer memory as required. Normally, the computer memory has a capacity of 1 M-bytes or less in view of fast operation. Assuming now that an aerial photography film of 23 cm.times.23 cm is processed with a 10 micron resolution for a photo survey, the external memory is required to contain about 500 M-bytes for storing the information regarding the whole area of the image. Since the image information regarding the necessary portion of the image is about 60% of the whole area of the image, the external memory is required to contain about 200 M-bytes for storing the information regarding the unnecessary portions of the image. If the digital computer is used to process 1000.times.1000 pixels (1 M-bytes) as the unit of an image on an aerial photography film, the computer memory is required to contain 1000.times.23000 bytes (23 M-bytes) for storing digital information regarding to 1000 scan line segments. Accordingly, the computer memory capacity required for one unit of the image increases by 23 times and the operation time required for one unit of the image increases by 23 times.